This invention relates generally to the purification of beet juice for subsequent sugar production, and in particular to a process for the removal of non-sugar impurities from raw juice.
In a typical commercial sugar beet processing operation, juice from the beets is extracted by means of a conventional diffusion process. This juice normally contains a considerable amount of beet tissue particles, commonly termed "mush," and usually about 13-16% water-soluble materials. The mush is separated by means of mechanical screening, and the remaining diffusion juice is further processed for purification and consequent sugar production. Prior to such purification, diffusion juice comprises water, sugar and various dissolved and colloidal non-sugar impurities. Because these non-sugars are usually highly-colorized, thermally-unstable material, they severely interfere with both the quality and quantity of subsequently-produced sugar. It is therefore most important to remove as great a quantity of non-sugars as possible from the diffusion juice.
A typical current process for such purification begins with a progressive preliming step where the pH of the juice is gradually increased from about 6.5 to about 11.6 by means of lime addition and carbonation sludge addition. Since many of the non-sugars are proteins, this gradual pH increase enables them to reach their respective iso-electric points, at which time they flocculate. After preliming, the resultant juice-floc-lime mixture enters a cold main liming step (30.degree.-40.degree. C.) and a hot main liming step (80.degree.-90.degree. C.) where additional lime is added to increase the pH to a value above about 12.6. This procedure acts to decompose non-sugars not affected during the preliming step, and is said to thereby stabilize the juice. The resulting limed juice composition is then subjected to a first carbonation step wherein carbon dioxide gas is introduced to react with the lime and produce a large volume of fine calcium carbonate precipitate. This calcium carbonate acts to adsorb some remaining dissolved non-sugars and additionally acts as a filter aid in the separation of prelimed flocs during settling in conventional sludge thickeners and during additional filtering. Finally, the resultant juice composition is subjected to additional heating, a second carbonation step, and final filtering. The resultant effluent, termed "thin juice," is then ready for evaporative thickening and sugar crystallization.
Upon investigation of the above-described process, however, it has been found that certain flocs formed during the preliming steps are sensitive and are destroyed because of the harsh mechanical and chemical conditions present during the main liming step. This destruction occurs because of repeptization of some of the proteinaceous flocs or because of mechanical degradation, and consequently lowers the effect of prelimer non-sugar separation. It is therefore a primary object of the present invention to provide a juice purification process wherein the non-sugar flocs formed during preliming and which separate certain non-sugar impurities from the juice remain relatively undisturbed for the remainder of the juice purification process. In accord therewith, another object of the invention is to remove these prelimer flocs from the juice prior to main liming, yet reclaim minor, yet significant, quantities of juice accompanying the prelimer flocs for subsequent processing. It is yet another object of the invention to reclaim such accompanying juice by re-introducing the prelimer flocs and accompanying juice to the remaining juice during the carbonation step of the purification process whereby calcium carbonate scales surround and protect respective floc particles. These and other objects will become apparent throughout the description which follows.